70 THE CIRCULATION OF NITROGEN IN NATURE. 
of nitrogen. Thus we regain the point at which our survey 
of the circulation of nitrogen commenced. 
Now a subject like the present naturally teems with 
interesting suggestions. I have chosen three matters 
arising out of it which are perhaps of exceptional interest, 
but which appeal to different sections of our Society. 
One of the most important problems of the day is the 
question of sewage disposal. In all the earlier attempts in 
this direction the utilization of the manurial matters con- 
tained in sewage was considered to be of as much importance 
as the removal of offensive substances. It was not long, 
however, before it became apparent to practical men that the 
recovery of the valuable constituents undoubtedly contained 
in sewage was impracticable owing to their extreme state of 
dilution, and that if sewage could be so far purified that it 
could be turned into natural watercourses without offence 
to the public weal, we ought to rest satisfied with* the good 
riddance of bad rubbish. To this end every imaginable 
kind of substance has been recommended, and generally 
patented, for the removal of the solid matter. I am not 
putting it too strongly when I say that the treatment of 
sewage by chemical precipitation is on its last legs, and has 
proved to be nothing better than a gigantic and costly 
failure. The establishment of sewage farms may be 
described as a blind groping in the right direction, for it 
implies an attempt to dispose of the organic matter of 
sewage by natural agencies. The impossibility, however, 
of setting aside a sufficient area of suitable land for the 
purpose has proved in most cases an insuperable obstacle to 
this device, and it is only lately, and as a direct result of 
the observations referred to earlier in this paper, that a 
move has been made in what is undoubtedly the right 
direction, that is, to provide a place where the purifying 
